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Is ragweed next to resist glyphosate?

Be it fate or selection — natural or otherwise — it appears common ragweed has become the latest glyphosate-resistant weed. Found in a 22-acre patch of north-central Arkansas dryland, no-till soybeans, the hardy ragweed has survived heavy, and repeated, shots of glyphosate. While lab work has yet to be completed, Extension researchers agree all indications point to resistance.

“We're not confirming anything, but we couldn't be more suspicious,” says Bob Scott, Arkansas Extension weed specialist. “To be honest, we hadn't expected to announce this before absolute verification. But we've seen so much evidence that even if these plants aren't resistant and we're able to kill them if we spray earlier, this is a problem weed because of its tolerance. With no-till crops everywhere, that's of major concern regardless.”

June 17, Randy Chlapecka took a call from a worried farmer just up the road from Newport, Ark. The producer reported some ragweed appeared unaffected after glyphosate applications. Chlapecka, Jackson County Extension agent, called Scott.

“This producer is not new to the farming game — he knows what he's doing and he does it well,” says Scott. “He pretty much sprays his crops automatically. So many days after planting, he puts the first glyphosate shot on and so many days after that, the second shot goes on.”

The field has been in a grain sorghum/soybean rotation for around a decade. Roundup has been used for that entire period.

The field is no-till, and there was a preplant application. It's uncertain if the ragweed emerged prior to that spraying. However, Scott says, the 1.5-pint June application had good coverage.

“We walked this field. We found scattered ragweed in various stages of control. Amid all the dead weeds, we found very healthy, lush ragweed.”

Scott says he's called out on similar problems regularly. In most fields it's usually misapplication, a spray boom set too high, the application rates too low — all kinds of things allowing escapes. But this field didn't appear to have any of those problems: the situation immediately set off alarm bells.

Except for a small section cordoned off for tests, another 2 quarts of Roundup was applied to the 22 acres. Two weeks later, Scott and Chlapecka checked the progress. Even when pulling into the field from the road, says Scott, “It was obvious the 2-quart application hadn't done what it should've.”


Surviving 1.5 pints of Roundup causes “a lot” of necrotic and burnt leaves. The common belief is that makes subsequent Roundup applications less likely to work. The plant has been toughened up and doesn't take the herbicide up as it should. That could be a factor here, says Scott, “but the plants looking as good as they do after sprayings is very odd.

“Right now, this ragweed actually looks like it's being segregated right in front of you. There are plants of all different sizes and health. The plants are stair-stepped and you'll see them dead as a doornail right beside ones that are bright green.”

After the 2-quart application didn't provide expected control, Scott and Chlapecka set up several tests. “On the (cordoned off) section, we went with 2, 4 and 8 quarts of Roundup Original MAX. In a section where the farmer had applied the 2 quarts, we put out an additional 2, 4 and 8 quarts. So, in this field today, we have ragweed with applications of 1.5 pints followed by 2 quarts followed by another 2 quarts, and they're still putting on seed. The 4- and 8-quart rates injured the soybeans but failed to completely control the ragweed.”

Researchers will evaluate the ragweed next year and spray earlier. “It's worth mentioning that for a 12-inch ragweed, the proper rate of Roundup is 22 ounces,” says Scott. “If the ragweed is 18 inches, the rate is 32 ounces. So if you spray a 12-inch ragweed using only a pint, you probably won't get a kill. A 1.5-pint rate may or may not kill a 12-inch ragweed.”

For that reason, Scott admits he's “a bit concerned that we're jumping the gun. But, by now, these weeds have had so much Roundup on them and there are so many plants in the field that look unaffected that it's beyond the theoretical. The plants might burn up a little and then will send out new growth. The bad thing is they're still producing seed.”

Monsanto is well aware of the problem ragweed, says Scott. The company is evaluating sample plants in St. Louis.

Chlapecka has investigated several other ragweed calls. None have proven worrisome.

“He thinks the ragweed in those instances was already very large when it was first sprayed,” says Scott. “Those fields are probably just experiencing partial control. The only thing that makes us believe that isn't what happened here is the various level of control. There are dead plants literally right beside plants that are green and full of seed.”


Ragweed is a very similar plant to marestail, a confirmed glyphosate-resistant weed. “They're not genetically similar,” says Scott, “but they are in growth habits and physiology. Like marestail, ragweed gets a woody stem and grows tougher with age. It also likes to grow in the same type of areas that marestail likes.”

If proven to exist, resistant ragweed isn't going to change agriculture profoundly. Ragweed isn't a huge problem in any major crop, and there are herbicides besides glyphosate to control the weed.

The larger issue, says Scott, is the path agriculture is traveling. “Two years ago, on the front page of Delta Farm Press was a headline saying ‘Horseweed is resistant.’ Now, two years later, all indications are we've got resistant ragweed. What's going to be next? Will it be pigweed?”

If, indeed, it is pigweed — as many experts believe — Scott says trouble looms. “I'll go on the record: it's a matter of when, not if, pigweed becomes resistant. Tall waterhemp or lambsquarter would also be bad news. In some areas — like Jackson County — where there's already Scepter/DNA-resistant pigweed, producers would be in a pickle because there's not another good option. If that scenario plays out, we'd be knocked back at least 20 years as far as herbicides.”

In the coming weeks, Scott and colleagues will be on the 22 acres harvesting ragweed seed and taking soil samples “to grow plants from the seed bank. We'll then grow the plants in the greenhouse and compare those to a population of ragweed that's definitely glyphosate-susceptible. We'll then run all kinds of tests in cooperation with Dick Oliver (weed biology specialist at the University of Arkansas) and Ron Talbert (resistance management specialist with the university). By Christmas, we should have a definite answer on this. I really hope this ragweed is just tolerant, but I wouldn't put any money on it.”


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